Fast-neutron nuclear reactors incorporate a large vessel having a vertical axis, filled with liquid metal up to a certain level and closed with a horizontal slab. The volume included between the top level of the liquid metal, which is generally sodium, and the slab is filled with an inert blanket gas, which may be argon. This argon, at a slight overpressure relative to the atmosphere, makes it possible to avoid any entry of air into the vessel of the nuclear reactor and any contact of the liquid sodium with an oxygen-containing gas.
The thick horizontal slab of the nuclear reactor consists of a metal casing filled with concrete. Inside this casing are arranged cylindrical passages having a vertical axis permitting the components of the reactor to be passed. These components are of very diverse sizes and constructions depending on their use in the vessel of the nuclear reactor. All these components have their lower part immersed in the liquid sodium which fills the vessel.
In particular, the primary pumps and the intermediate exchangers of the reactor are formed in the shape of wide-diameter components of a generally cylindrical shape and great length, arranged in the vessel with their axis vertical.
All these components incorporate in their top part a bearing flange which rests on the reactor slab to hold the component in an operating position in the reactor vessel.
The component passages which are arranged through the slab have an inner diameter which is appreciably greater than the diameter of the component part which is at the level of this passage in its working position. An annular space of a relatively large width is thus provided between the component and the slab.
Some components such as the intermediate exchangers, have their outer wall, in the region of the passage through the slab, at a very high temperature when the reactor is operating. An intermediate exchanger, in fact, forms an enclosure inside which secondary liquid sodium at a high temperature, above 500.degree. C., circulates upwards in contact with the outer wall of the exchanger.
Other components such as pumps, on the other hand, have a wall the temperature of which remains relatively moderate in the zone of the passage through the slab. In all cases, however, the outer surface of the component whose lower part is immersed in the liquid sodium is at a temperature which is considerably higher than the temperature of the slab, which is in the region of 100.degree., at the level of the passages for the components.
Heat exchange phenomena are therefore produced between the components and the reactor slab, in the annular space which exists between the component and the passage.
These heat exchanges by radiation and by convection produce locally heating of the slab structure and, more particularly, of the passage, which gives rise to internal stresses in this slab and can cause distortions of its structure. Furthermore, the inert blanket gas entering the annular spaces between the components and the slab tends to increase the thermal exchanges, while promoting the formation of hot zones in the passages through the slab, by local circulation between the component and the passage. It would obviously be desirable to prevent this appearance of high-temperature zones and to obtain as uniform a temperature as possible in the passages through the slab by channelling the flow of the blanket gas in the annular spaces between the components and the slab.